Reactions of Co(NO)D2 (D equals monoantion of dimethylglyoxime), (CoC1(NO)(en)2)C104 (en equals ethylenediamine), and black (Co(NH3)5NO)C12 with hemoproteins that are capable of forming stable nitrosyl complexes have been investigated. Selective intermolecular transfer of the nitrosyl ligand in Co(NO)D2 and (CoCq(NO)(en)2)C104 to hemoglobin A and myoglobin has been observed. In contrast, black (Co(NH3)5NO)C12 rapidly transforms hemoglobin, methemoglobin, myoglobin, metmyoglobin, and iron (II) and iron(III) horseradish peroxidase hemoproteins, and ferricytochrome c to their corresponding nitrosyl complexes through initial decomposition of the cobalt nitrosyl complex with water to liberate nitric oxide quantitatively. The oxidation of deoxyhemoglobin to methemoglobin by nitric oxide has been investigated. This unexpected process appears to involve hydrogen transfer from the thiol group of the cysteine amino acid at the 93-position in the beta-chains of hemoglobin and implicates nitrosyl hydride as a potentially significant reactive chemical intermediate. Complimentary studies of reactions of oxyhemoglobin and oxymyoglobin with nitric oxide that result in the formation of the iron (III) hemoproteins and nitrate define this process as involving the oxygen transported hemoproteins as "superoxide" donors. Oxidation of hemoglobin to methemoglobin with liberation of nitric oxide characterizes the behavior of alkyl nitrites, nitrosoguanidines, and N-nitrosodiphenylamine with this hemoprotein. Reactions of dinitrogen trioxide and dinitrogen tetroxide are more complex, but can be understood through component investigations of transformations caused by nitrites and by nitric oxide.